TM 5-810-15
up to 20 psig for the conveying system as shown in
system, reduced to one transfer point with a
figure 6-6. System capacity and long conveyor
minimum of controls, then delivers collected ash to
distances sometimes require higher blower
any terminal point at a distance of several thousand
pressures. Pressure systems may be used in lieu of
feet. The vacuum pressure system provides the
vacuum systems because of higher capacities or
least complex controls of any long distance
longer conveying distances. Pressure type system
pneumatic conveying system.
d. Ash storage silos. Storage silos are usually
should be avoided where possible because leaks of
fine ash particles usually occur at the piping joints.
constructed of carbon steel because of its lower
Silo storage design is the same for a pressure
cost and durability. Hollow concrete stave con-
system as for a vacuum system except that ash
struction or reinforced concrete construction are
collectors are not required at the silo and fly ash is
sometimes used. The bottom of ash storage silos
redeposited directly into the silo. There are two
are equipped with aeration stones to fluidize the
types of pressure systems, the dilute phase and
ash and induce flow from the silo to the discharge
dense phase. The dilute phase system usually has an
outlets. Silos will be designed for a minimum of
ash to air volumetric ratio of 15 to 1 and sometimes
sixty hours of storage, based on the design and
it is as high as 30 to 1. A dense phase system has an
production rate, utilizing an ash density of 60 pcf.
ash to air ratio of 40 to 50 to 1 and is sometimes as
The actual ash density can vary from 60 pcf
high as 80 to 1. Vacuum systems are classified as
depending on the coals being fired. The silo sup-
dilute phase. A comparison of pressure dilute phase
port structure will be designed for a full silo with
and dense phase systems is shown in table 6-2. The
fly ash density 100 pcf.
dilute phase pressure system is the more widely
used pressure system. Dense phase pressure
6-6. Controls
systems utilize a fluidizing transporter, a vessel in
a. General. Programmable type control systems
which air and ash is mixed, fluidizing the ash so
are used for both automatic and semi-automatic
that flow characteristics resemble that of a liquid.
control. Older systems used electromechanical type
c. Vacuum/pressure systems. In some rare cases,
control systems, many of which are still in opera-
it may be more economical to combine a vacuum
tion.
system with a pressure system where distance rules
b. Types.
out the use of a vacuum system alone. Figure 6-7
(1) Programmable controllers (PC) have
shows a typical vacuum/pressure system. The
been applied to ash handling systems with good
vacuum system, with its simplified controls,
success during the last fifteen years and are the
removes ash at an optimum rate. The pressure
6-5
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